1. Field of the Invention
This invention relates to inertial measurement and navigation systems and more specifically to the use of digital signal processing techniques with an inertial multisensor.
2. Description of the Prior Art
Commercial aircraft flown today often employ directional and vertical gyroscopes to measure the pitch, roll and heading of the aircraft. Newer systems, called Inertial Navigation Systems, employ expensive precision gyroscopes and accelerometer to calculate global positions based on a star fixed reference. Inertial sensing techniques require the measurement of three axes of rate information and three axes of acceleration information. Typical sensors provide only rate or acceleration information and sometimes for only one axis. Rate measurement information provided by various gyroscopic devices, often called gyros, such as conventional gimballed gyroscopes, and tuned rotor gyros can provide two axes of rate information. Other types of gyroscopes, like the Ring Laser Gyro and Fiber Optic Gyro generally provide only one axis of information thus requiring three sensors. Most acceleration sensors provide only one axis of information, again requiring three sensors. A more economical non-navigational inertial strapdown system, called an Attitude and Heading Reference System (AHRS), is also used primarily in small aircraft. The trend in general aviation is toward strapdown sensor technology in a form of AHRS which requires small and low cost rate and acceleration sensors. Although the AHRS is relatively inexpensive compared with full navigation systems, they are more expensive than gimballed, directional and vertical gyroscopes used general aviation aircraft.
New sensors, called multisensors, have emerged in recent years that are less expensive than the gimballed gyroscopes, yet provide sufficient quality for use in an AHRS. These prior art multisensors are described in the following brochures: "A Low-Cost, Multifunction Inertial Sensor for Tactical Applications", by I. Pick The Singer Company and "Low-Cost Piezoelectric rate/acceleration Sensors", by B. F. Rider, G. L. Vick, J. S. Hunter and A. Rodgers for Recoil International Corporations Avionics Group. Multisensors of these two types which measure two axes of rate information and two axes of acceleration information are presently available. These multisensors are attractive because they are capable of measuring two axes of rate information and two axes of acceleration information. However, one is relatively large and heavy and the other is lighter but uses slip rings to extract information. Because slip rings wear out relatively quickly, there's a short Mean Time Between Failure.
A typical AHRS system often uses tuned rotor gyros and servoed accelerometers. Such a system typically uses two gyros and three accelerometers for total cluster of five sensors. The tuned rotor gyros used in a typical AHRS operate in a closed feedback loop using either analog torquing loops or digital torquing loops. Most implementations today use some variation of conventional demodulation techniques for signal extraction. Conventional demodulation techniques involve mixing the analog input information signals down to a base-band, filtering to remove second harmonics and compensating for noise sources. Conventional techniques involve significant analog circuitry that is subject to inherent error from temperature sensitivity, analog noise sources and induced pick-up such as Electromagnetic Interference (EMI). The present invention solves a problem in using a new multisensor sensor by teaching a highly accurate yet cost effective, methods of extracting the sensors rate and acceleration information.